The 26-year-old Oscar nominee debuted her brighter look at Comic Con on Thursday.
"I took Carey from a medium brown to a gorgeous, soft blonde," celebrity colorist Negin Zand tells Us Weekly. "She wanted something more natural than the platinum color we've seen on her before and it really suits her. It looks amazing with her new, short cut!"
Mulligan — who first went blonde for a film role in 2009 — told January's ELLE UK she prefers to be a brunette.
"I hate my hair. After An Education, I had a small part in Michael Mann's film Public Enemies and he wanted me to dye it white blonde, and it wrecked my hair," Mulligan complained. "Literally, ruined it. It was falling apart, fried. So I had to cut it off. I cried."
Her latest shade switch is also for a movie role.
Tell Us: Do you like Carey better as a blonde or brunette?
For access to all our exclusive celebrity videos and interviews – Subscribe on YouTube!